1. Field of the Invention
The present invention is related a method for preparation of a Zn--Ni alloy electroplating bath using a Zn--Ni alloy. Such an alloy is used, for example, for simultaneously dissolving Zn and Ni into a Zn--Ni electroplating bath, which in turn is used for continuously producing a Zn--Ni electroplated steel sheet by utilizing an insoluble anode. In addition, the present invention is related to a method for simultaneously dissolving Zn and Ni into a Zn--Ni hot-dip galvanizing bath. Furthermore, the present invention is related to a method for producing a Zn--Ni alloy.
2. Description of Related Arts
There are two methods for supplying metals into an acidic Ni--Zn electroplating bath using an insoluble anode.
(1) A method for supplying the metals into the plating bath by means of dissolving the metals in the form of a soluble salt, such as a basic carbonate.
(2) A method for supplying the metals into the plating bath by means of separately bringing the plating metals, i.e., Ni and Zn, into direct contact with the acid of the plating bath.
Method (1) is superior to the Method (2) with respect to dissolving performance. Method (1) is, however, inferior to Method (2) in cost. Method (2) is cost-effective but its poor dissolving performance is a disadvantage.
Dissolving Ni and Zn in the acidic solution by Method (2) involves a cathodic reaction 2H.sup.+ +2e=H.sub.2. However, since the hydrogen overvoltage of zinc is high and this makes it for the above reaction to take place. This seems to be a reason for the poor dissolving performance of the method (2). Particularly, the Zn dissolving performance is impaired also by Ni.sup.2+ ions present in the acidic plating bath, because Ni.sup.2+ ions replace for Zn the metallic Zn and then precipitate on the metallic surface. The metallic Zn is therefore covered with the Ni, so that the dissolving of Zn is impeded.
Under the circumstances of the prior technique described above, prior art does not simultaneously dissolve the metallic Zn and Ni from the same source.
Japanese Unexamined Patent Publication No. 60-248855 discloses a Zn--Ni alloy, with 3% or less of Ni used for preparation of a hot-dip galvanizing bath. This publication describes that a Zn--Ni alloy with a higher Ni content causes vigorous vaporization of Zn as the Zn--Ni alloy is dissolved, and more Ni is transferred into dross than when Zn--Ni alloy with less than 3% of Ni is dissolved. Incidentally, the zinc metal is melted and then Ni is added to the molten Zn so as to provide an alloy having a predetermined composition.
The following methods are known heretofore for producing a Zn--Ni alloy.
(1) Metallic Zn and metallic Ni are melted to produce a Zn--Ni alloy.
(2) Ni salt, for example, nickel chloride, is added to the metallic Zn.
Zn--Ni alloy with 2 wt % or less of Ni has a melting point of approximately 600.degree. C. Such Zn--Ni alloy can therefore be melted without relying on a flux. However, since the melting point is greately raised when the Ni content is higher than 2 wt % according to a phase diagram, the melting temperature of Zn--Ni alloy exceeds the temperature where vigorous vaporization of Zn occurs. It is therefore extremely difficult to produce a Zn--Ni alloy by melting. More specifically, when the surface temperature of Zn--Ni bath exceeds 750.degree. C., the Zn vigorously vaporizes and is oxidized. As a result, an ignition and combustion phenomenon occurs. In addition, bumping phenomenon of the Zn--Ni bath may occur. For the reasons described above, it is recognized that production of Zn-high Ni alloy is difficult by Method (1).
In Method (2) also, a high temperature is necessary for producing a Zn--Ni alloy. In addition, since nickel chloride, which is expensive, is used in Method (2), this Method is not advisable.